The removal of air borne particulate contaminants from the air is a concern to everyone. Gas phase particulate filtration has traditionally been accomplished by methods that utilize woven or nonwoven fabrics or webs. The performance of such a system is characterized by the initial efficiency of removal or capture of the particulate as a function of particle size, the initial resistance of the system to air or gas flow as a function of gas flow rate or face velocity, and the way both of these factors change as the filter element loads with the particulate contaminant. One common measurement is the alpha value of a media, which is the product of the pressure drop and the filtration efficiency, and is calculated as follows:alpha=−100*log((100−efficiency)/100)/Pressure Drop.
Generally, it is desirable that a particular filter media have a higher alpha value, as this is indicative that media has a low pressure drop and a high efficiency. For example, the glass materials that are used for ASHRAE bag filters have alpha values (obtained with a DOP challenge) that are in the range of 12-16 (depending upon the particular efficiency of the media), and are not reliant on any type of electrostatic charge to achieve this alpha value. Glass paper can have alpha values of about 12-13, membrane materials can have alpha values of about 20, and nanofiber electrospun materials can have alpha values in the range of about 5-12. None of these materials is reliant on any type of electrostatic charge to achieve these alpha values.
Filtration media formed from using meltblown, spunbond, carded nonwoven, and wet laid synthetic materials can have very high alpha values when they are electrostatically charged. However, when the charge is removed, the alpha values of these media significantly decreases to levels that are well below the alpha values of media made using other materials.
Accordingly, there remains a need to provide an improved filter, and more particularly to provide filter media and filter elements having improved discharged alpha value.